The present invention relates generally to battery chargers, and more specifically to a charging circuit for a battery-powered portable apparatus such as portable personal computers.
Prior art battery-powered apparatus such as portable personal computers are provided with an AC-DC converter or adapter for charging the rechargeable battery of the apparatus with commercial power. The prior art charging circuit includes a trickle charging circuit and a speed charging circuit. The trickle charging circuit is switched into circuit with the battery when the initial battery voltage is lower than a reference level, so that the battery is charged with a low current to gradually raise its voltage. This initial low current charging is to avoid a serious damage on the battery if the battery is charged when its voltage is too low. As the reference voltage is reached, the speed charging circuit is then switched into circuit with the battery to charge it with a high current for speed charging. As the battery voltage reaches a known peak value, the charging circuit terminates the speed charging mode and sets the trickle charging circuit again into operation, so that the battery is continuously charged with a low current. This trickle charging mode is continued until the AC-DC adapter is disconnected from the apparatus.
One problem inherent in the prior art is that, if the AC-DC adapter remains connected to the apparatus, the trickle charging mode is indefinitely continued, and will cause an overcharging of the battery, which will result in a shortening of the battery's lifetime.